Cleaning aluminum workpieces

ABSTRACT

A method of cleaning an Al workpiece comprises a.c. anodising the workpiece in an acidic electrolyte capable of dissolving aluminium oxide and maintained at a temperature of at least 70° C. under conditions such that the surface of the workpiece is cleaned with any oxide film thereon being non-porous and no more than about 20 nm thick.

This application is a 371 of PCT/GB95/02956 filed Dec. 18, 1995.

There is a considerable volume of data on the cleaning of aluminiumworkpieces prior to subsequent surface finishing treatments. Some ofthese are only suitable for batch production as a precursor to, forexample, architectural anodising and are not fast enough for continuoushigh speed operation. A good overview is given in "The Surface Treatmentand Finishing of Aluminium and its Alloys" by S Wernick, R Pinner and PG Sheasby, Finishing Publications Ltd, 1987, Teddington, U.K.

Generally aluminium surfaces are cleaned using acid or alkalinesolutions. Alkaline etching solutions are faster than acid ones and tendto cope well with residual organics on the surface of the workpiece.Unfortunately, they do not dissolve the magnesium oxides left on thesurface of magnesium containing alloys that have been thermally treated.They also often require an acidic desmutting step and very carefulrinsing control, and deposits build up rapidly in the bath. The fastestacidic cleaners contain hydrofluoric acid plus another acid such assulphuric acid. Such known treatments are capable of removing materialat rates up to about 1 g/m² /min.

In U.S. Pat. No. 3,718,547, W E Cooke et al. describe a high speedcontinuous electrolytic surface cleaning treatment of aluminium strip.In a preferred embodiment, the strip is made successively cathodic,anodic and finally cathodic again while being subjected to d.c.electrolysis in a sulphuric acid electrolyte at 90° C. This treatmentresults in the formation of an anodic oxide film quoted as being 5 to 50mg per 100 square inches (which corresponds to a film thickness of30-300 nm assuming an oxide density of 2.5 g/cm³) and which forms anexcellent base for lacquer.

In U.S. Pat. No. 4,097,342, W E Cooke et al describe an electrolyticcleaning treatment step which involves subjecting aluminium strip tod.c. anodising for a few seconds at high temperature and current densityin a concentrated strong mineral acid electrolyte.

The present invention provides a method of cleaning an Al or Al alloyworkpiece which method comprises anodising the workpiece using a chosena.c. voltage X (expressed in rms V) in an acidic electrolyte capable ofdissolving aluminium oxide and maintained at a temperature of at least70° C. under conditions such that the surface of the workpiece iscleaned with any oxide film thereon being non-porous and having athickness Y (expressed in nm) wherein Y is not more than about half X,or a thickness of not more than about 20 nm. Preferably the cleaningtreatment consists essentially of this step, i.e. without any otherspecial steps being necessary. The following technical explanation maybe of interest.

Anodising, whether a.c. or d.c., can produce a wide range of oxide filmstructures. The type of structure produced is generally dependent on thevoltage applied across the film at the surface and the aggressiveness ofthe electrolyte. Thus in a non-aggressive electrolyte only a barrierfilm is grown that reaches a limiting thickness governed by the voltageapplied, i.e. a limiting field is achieved that will no longer driveions through the film. However, if the electrolyte can dissolve the filmthen, once the normal barrier film thickness is achieved, cells areformed on the surface that each have a pore in the centre. The oxidefilm at the base of these pores continues to grow into the metal and bedissolved rapidly at the electrolyte-film interface thus maintaining thebarrier film thickness. Dissolution at the base of the pores is greatlyenhanced over the normal chemical dissolution rate by the electric fieldwhich results in the columns of oxide between the bases of the poresbeing left unattacked or `growing` to form the cell walls. In anaggressive acid, such as sulphuric or phosphoric acid, the structureformed is strongly dependent on the temperature and acid concentration.Thus at room temperature the dissolution in the pore is so slow that lowcurrents are used and films can be made many microns thick without theoriginal outer surface being significantly attacked, e.g. architecturalfinishes and films of the kind described in EP 0178831 are produced atlow temperatures. At higher temperatures only thin films can be grownbefore the outer surface is attacked, however, these films can be grownvery rapidly as the dissolution in the pores is considerable; this isused to advantage when high speed anodising to pretreat metal stripssuch as the processes described in EP 0181183. The pores in these filmstend to be more open and in extreme cases adjacent pores will mergeleaving only filaments of the pore wall behind. This is commonly seen inphosphoric acid films used for pretreatment.

If the acid is made even more aggressive then a point is reached atwhich the rate of film dissolution is greater than the rate of formationand a `bare` surface results. However, as the rate of film dissolutionis electric field enhanced the speed of etching is very fast indeed andthe process lends itself to high speed cleaning volume production. Inaddition, when a.c. power is employed copious quantities of hydrogen areevolved in the cathodic half cycle and smut (whether deriving fromalloying elements, e.g. silicon or copper, metal fines or organicresidues) is blown off the surface leaving a surface that is cleanerthan just pickling in the hot acid would achieve.

Aluminium metal in air carries a naturally occurring oxide film some 2.5nm thick at room temperature. The barrier layer formed when Al isanodised in a non-aggressive electrolyte has a limiting thickness(expressed in nm) of some 1.0 to 1.4 times the anodising voltage. Thecleaning method of this invention is generally performed underconditions such that any oxide film on the surface of the workpiece atthe end of the treatment is no more than about half the barrier layerthickness that might have been predicted using this formula from theanodising voltage employed. Preferably any residual oxide film is lessthan 10 nm thick, e.g. less than 2.5 nm thick. Thus any oxide film onthe surface of the Al workpiece at the end of the cleaning treatment isvery thin.

The cleaning method can be carried out in conventional baths used (underdifferent conditions particularly lower electrolyte temperatures) fora.c. anodising. In an a.c. treatment, it is envisaged that a surfaceanodic oxide film is grown during the anodic part of the cycle.Dissolution occurs during both parts of the cycle and an equilibrium isset up whereby the rates of growth and dissolution are the same and thebarrier thickness of any anodic oxide film remains constant. It isthought likely, though not certain, that a thin anodic oxide film isalways present. A graph of current density against time for a.c.anodising at constant voltage suggests that this equilibrium is reachedin 0.3 to 3.0 s. When a.c. is used with graphite counter-electrodes thefrequency is preferably greater than 25 Hz. Other inert or noble metalsor metal oxides can be used as counter-electrodes.

The temperature at which the rate of film dissolution is greater thanthe rate of formation, so that a.c. anodising effectively cleans thesurface, is always at least 70° C. usually at least 75° C. But in anyparticular case the minimum temperature required to achieve thistechnical effect is dependent on a number of factors:

The nature of the acidic electrolyte. This electrolyte must always beone having some dissolving power for aluminium oxide. Phosphoric acidand sulphuric acid-based electrolytes are preferred. Phosphoric acidelectrolytes are chemically more aggressive and minimum cleaningtemperatures for commonly used alloys are lower, e.g. in the range of 80to 95° C. Minimum cleaning temperatures for commonly used alloys insulphuric acid are typically 92 to 96° C. Mixed acid electrolytes arenot preferred, on account of the difficulty of recycling/regeneratingsuch mixtures.

The term phosphoric acid is here used to cover a family of related acidsbased on various phosphorus oxides. This family includes orthophosphoricacid H₃ PO₄, metaphosphoric acid and pyrophosphoric acid based on P₂ O₅; and also phosphorous or phosphonic acid H₃ PO₃ ; hypophosphorous orphosphinic acid H₃ PO₂ ; and perhaps others. As electrolytes withdissolving power for aluminium oxide they all have generally similarproperties, and are here included under the generic name phosphoricacid.

The term Al is herein used to denote pure aluminium metal and alloyscontaining a major proportion of aluminium. The nature of the Al alloyis not material to the invention. But the composition of the Al alloy,and particularly the Mg content, does have a material effect on theminimum cleaning temperature. This can be illustrated by reference tothe automotive alloys AA6111 and AA5754 (of The Aluminum AssociationInc. Register of April 1991). In contrast to AA1050A lithographic sheet,these materials contain magnesium at 0.5-1.0 wt % and 2.6-3.6 wt %respectively. This has two significant effects. Firstly the surfacefinish after rolling of these materials is much more broken up due tothe presence on the surface of mixed aluminium and magnesium oxides andalloying metal. This is caused by a thick magnesium oxide film growingon the surface of the ingot during homogenisation which in turn causesexcessive `pick up` during hot rolling. These picked-up metal/oxideparticles are redeposited on the strip during rolling. The thickness ofthese particles is up to approximately 1 micron for 6111 and 2.5 micronsfor 5754 and for many subsequent operations they have to be at leastpartially removed. In order to clean these materials a higher currentdensity, e.g. 2-5 kAm⁻², is required than for lithographic sheet, inorder to achieve the necessary surface removal in an acceptably shorttime for a continuous process.

The second major effect of the magnesium content of the alloy is that itstrongly affects the rate of dissolution. Consequently under anodisingconditions the film growth rate is faster for higher magnesiumcontaining alloys but the barrier film is thinner under identicalconditions.

There is no sharp cut-off point at which dissolution exceeds film growthrate. The strong factors are temperature and magnesium content of thealloy. Also important but lesser influences within the window ofconditions that are desirable for continuous operation are:

Acid concentration. Phosphoric and sulphuric acid concentration ispreferably 5-35% by weight, e.g. 15-25%. Aluminium content of theelectrolyte should preferably be kept below 10 g/l (of Al ion) inphosphoric acid electrolytes and below 20 g/l in sulphuric acid, sincehigher levels may cause a damaging decrease in conductivity.

Wave form type. The wave form may be sinusoidal or not as desired.Although deliberate bias is not preferred, the a.c. current may bebiased in either the cathodic or anodic direction. The a.c. frequency isat least several cycles per second and is preferably the commercialfrequency.

Voltage. A.C. voltages expressed herein are rms voltages measured(unless otherwise stated) at the workpiece. Particularly in commercialoperation, voltage of the power source may be significantly higher thanthis. While the potential across the surface of the workpiece isimportant, it is in practice often easier to measure the voltage at thepower source. Preferred voltages (at the power source) are in the rangeof 0.5-100 volts. Below 50 V, the risk to users is reduced. At ananodising voltage of 20 V (at the workpiece), any oxide film remainingon the surface of the cleaned Al workpiece is expected to be not morethan 10 nm thick.

It is generally easier to monitor current density rather than voltage.Although the relationship between the two depends on the equipment beingused, the following relationship has been found useful in the inventorslaboratory. A current density of N kAm-⁻² often corresponds to an a.c.anodising voltage of about 4N to 6N V.

Preferred current densities are in the range of 0.1-10 kAm⁻². As notedabove, the higher current densities may be required for alloyscontaining Mg. When higher current densities are used, minimum cleaningtemperatures are generally higher for any given alloy.

As shown in the examples below, the cleaning method of this invention iscapable of removing material from the Al workpiece at a rate of 5.5-10.5g/m² /min. This is some 5.5-10.5 times faster than is achieved in anyexisting acidic cleaning process. This advantage is particularlyvaluable when the workpiece is an Al sheet or strip which is subjectedto rapid continuous cleaning by immersion in electrolyte for a shortperiod e.g. 0.1-10 seconds.

The processes occurring when using a.c. are:

i) Cathodic gassing (2H⁺ +2e⁻ →H₂) that cleans loose detritus off thesurface. A demonstration of this is to immerse AA6111 alloy in hotphosphoric acid without power applied. The dissolving surface leavesbehind a copper containing black smut. Application of power will removethis or if the surface was not immersed for long before power wasapplied the smut does not have time to form.

ii) Field enhanced chemical dissolution. This occurs in both the anodicand cathodic cycles. The presence of a field stretches Al--O bonds andallows for easier attack.

iii) Film growth which of course occurs in the anodic cycle.

So in the anodic cycle ii) and iii) compete and naturally greaterdissolution is expected in the cathodic cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is directed to the accompanying drawings in which:

FIGS. 1(a) and (b) comprise two graphs shown as (a) and (b) illustratingthe surface concentrations of oxygen and magnesium (as measured byelectron microprobe) for AA6111 electrolytically cleaned at (a) 80° C.and (b) 90° C.

FIGS. 2(a) and (b) consist of two corresponding graphs for AA5754 alloy.

FIG. 3 is a graph of barrier layer thickness measurements for AA5754 andAA6111, electrolytically cleaned for 1, 2, 3 and 6 seconds.

FIG. 4 is a graph to show actual film growth against anodising voltage(a.c.) for 1050A (0.3 mm) at different temperatures in 20% H₃ PO₄.

FIG. 5 is a graph to show actual film growth against anodising voltage(a.c.) for 5182 (0.3 mm) at different temperatures in 20% H₃ PO₄.

FIG. 6 is a graph to show actual film growth against anodising voltage(a.c.) for 1050A (0.3 mm) at different temperatures in 2.04 molar H₃PO₃.

FIG. 7 is a graph to show actual film growth against anodising voltage(a.c.) for 5182 (0.3 mm) at different temperatures in 2.04 molar H₃ PO₃.

FIG. 8 is a graph to show actual film growth against anodising voltage(a.c.) for 5182 (0.3 mm) at different temperatures in 2.04 molar H₂ SO₄.

The following Examples illustrate the invention.

EXAMPLE 1

A commercial anodising plant was operated under the following conditionsfor cleaning lithographic sheet (AA1050A). The conditions were:

Acidic strength--20 wt % phosphoric acid

Time (under electrodes)--0.4-1.0s

Temperature--85° C.

Current density--1 kAm⁻² (a.c.)

Voltage--about 20 V (a.c.) at the power supply.

The resulting surface finish has been the subject of a study which hasshown that the surfaces produced are as free of organic contaminants asany industrial finish examined to date, and have a thinner film on thesurface than the natural oxide thickness. Consequently over the twoweeks following cleaning this film thickens up to the natural thicknessof 2.5 nm.

EXAMPLE 2

Sheet samples of 0.3 mm gauge AA1050A were treated in a 20 wt %phosphoric acid solution at a current density of 3 kAm² a.c. for 5 s atvarious temperatures. This alloy was chosen as it has a very low levelof magnesium and therefore the threshold temperature at whichdissolution begins to exceed anodic film growth should be at itsmaximum. At 80° C. a porous anodic film was formed on the surface but at85° C. only a thin barrier film was produced indicating that thelimiting barrier film thickness was not attained for the current densityemployed.

EXAMPLE3

As noted above, there is not a sharp cut-off point at which dissolutionexceeds film growth rate. However at commercially relevant currentdensities, control of the growth of a filamented anodic oxide film wouldbe difficult much above 70° C. especially on a high magnesium alloy,while reliable cleaning with respect to obtaining a thin film on thesurface may require temperatures of at least 85° C. For high magnesiumalloys a temperature as low as 80° C. may be practically possible. Thuscommercially pure material such as AA1050A lithographic sheet requires85° C. (see Example 2), as does AA6111, for even though it has somemagnesium in the alloy it also requires a higher current density toobtain rapid cleaning and will grow a film at 80° C.

Two different alloys were subjected to electrolytic cleaning by themethod of this invention in laboratory equipment under the followingconditions:

Acid strength--20 wt % phosphoric acid

Time--1-6s

Temperature--80° C. or 90° C.

Current density--5 kA/m² a.c.

Voltage--approximately 20 V at 80° C. and 15 V at 90° C.

Results for AA6111 alloy are shown in FIG. 1. Graph (a) shows surfaceconcentrations of four elements, determined by electron probe areaanalysis, after electrolytic cleaning at 80° C. for 1 to 6 s. Thesignificant reading for oxygen indicates the presence of an anodic oxidefilm of significant thickness.

By contrast, Graph (b) shows the results obtained after electrolyticcleaning at 90° C. The absence of oxygen indicates that an oxide filmwas either absent or was present only at very low thickness.

FIG. 2 shows comparable results for 5754 alloy. At both 80° C. and 90°C., the method was effective to electrolytically clean the surface ofthe workpiece.

FIG. 3 is a graph showing barrier layer thickness a.c. impedancemeasurements of the same cleaned surfaces as in FIGS. 1 and 2, namelyAA5754 cleaned at 80° C. and 90° C., and AA6111 treated at 80° C. and90° C. The AA6111 sample which had been treated at 80° C. had a residualoxide layer more than 10 nm thick. The other three samples had residualbarrier layers less than 5 nm thick.

EXAMPLE4

The same alloys AA5754 and AA6111 were a.c. electrolytically cleaned in20 wt % phosphoric acid in laboratory equipment for 2 minutes. Thecleaning conditions and the results obtained are set out in Table 1.Voltage figures were measured at the electrodes of the tank. Attentionis directed to the column headed "weight loss" where the figures aresome 5 to 10 times the size of any achieved previously in acid cleaning.

                  TABLE 1                                                         ______________________________________                                        Effect of Prolonged Phosphoric Acid Cleaning on Substrate                     Weight Loss and Surface Carbon                                                      Current  Bath           Weight  Surface                                       Density  Voltage Temp.  Loss    Carbon                                  Alloy kA/m.sup.2                                                                             (V)     (° C.)                                                                        (g/m.sup.2 /min)                                                                      (mg/m.sup.2)                            ______________________________________                                        5754  5        20      80      9.51 ± 0.18                                                                       1.62 ± 0.31                                         15      90     10.31 ± 0.22                                                                       1.29 ± 0.31                          6111  3        12.5    80      5.5 ± 0.16                                                                        1.05 ± 0.24                                         10      90      5.82 ± 0.16                                                                       1.09 ± 0.22                          ______________________________________                                    

EXAMPLE 5

Commercial anodising equipment using a sulphuric acid electrolyte wasoperated under different conditions to electrolytically clean AA8011closure stock. The conditions used were:

Acid strength--18 wt % sulphuric acid

Time (in bath)--3s

Current density--2 kAm⁻² (a.c.)

Voltage--6 V at the power supply.

The temperature was varied, and it was found that there was a quiterapid switch-over from anodising to cleaning at temperatures above 90°C. A temperature of 95° C. was chosen as the minimum effective cleaningtemperature under these conditions for this alloy.

EXAMPLE 6

Some other AA6000 series experimental materials were treated underconditions that were shown to produce a thin barrier film on 6111, (seeExample 3). These were:

Acid Strength--20 wt % phosphoric acid

Time 3s

Temperature--90° C.

Current Density--2 and 3 kA/m² a.c.

Voltage--approximately 7 V and 10 V (for 2 and 3 kAm⁻² respectively)measured at the tanks electrodes.

The alloys employed were AA6009 and two variants of AA6016, namely a lowcopper variant (0.01%), labelled 6016A, and a medium specification rangecopper variant (0.1%), labelled 6016B and having the characteristics:

    ______________________________________                                                                                  Grain Size                          Cu        Fe      Mg     Mn    Si   Ti    μm                               ______________________________________                                        6016A  0.01   0.28    0.42 0.08  1.17 0.01  21 × 32                     6016B  0.10   0.29    0.40 0.08  1.22 0.01  22 × 32                     ______________________________________                                    

Process Route

Homogenise 18h 560° C. (4h)

Hot Roll 5.0 mm (335° C.)

Cold Roll 1.2 mm (76%)

CASH anneal 540° C.

The following film thicknesses (in nm) were found after treatment:

    ______________________________________                                        Alloy          2 kA/m.sup.2                                                                           3 kA/m.sup.2                                          ______________________________________                                        6009           5        6                                                     6016A          6        5                                                     6016B          6        5                                                     ______________________________________                                    

All these films are regarded as thin.

EXAMPLE 7

Pairs of samples of 1050A and 5182 were connected across an a.c. powersupply and anodised against each other in 20 wt % phosphoric acid atvarious voltages and temperatures. The voltages were measured at theworkpiece. The run length was 10 s. After this the samples weresubjected to a.c. impedance measurement to determine the steady statebarrier layer.

FIG. 4 shows the barrier film growth of 1050A. The films generally arethinner at lower voltage and higher temperature. The cleaning treatmentsperformed at 80° C. and above are in accordance with this invention,while those performed at lower temperatures are not.

FIG. 5 shows the barrier film growth for 5182 under similar conditions.The film thicknesses are generally less than their 1050A counterparts.Cleaning treatments performed at 90° and 95° C. are in accordance withthe present invention.

EXAMPLE 8

This was performed as described in Example 7, except that the acid waschanged to 20 wt % phosphonic acid (phosphorous acid).

FIG. 6 shows the film growth for 1050A and FIG. 7 shows the film growthfor 5182.

EXAMPLE 9

This was performed as described in Example 7, except that the acid waschanged to 20 wt % sulphuric acid. FIG. 8 shows film growth for 5182.

We claim:
 1. A method of cleaning an Al or Al alloy workpiece whichmethod comprises anodising the workpiece at a chosen a.c. voltage X(expressed in rms V) in an acidic electrolyte containing phosphoric acidor sulphuric acid capable of dissolving aluminium oxide and maintainedat a temperature of at least 80° C. for high magnesium alloys and atleast 85° C. for all other alloys under conditions such that the surfaceof the workpiece is cleaned with any oxide film thereon being non-porousand having a thickness Y (expressed in nm) wherein Y is not more thanabout half X.
 2. A method as claimed in claim 1, wherein the anodisingis continued until an equilibrium is reached between oxide filmformation and dissolution.
 3. A method as claimed in claim 1, whereinthe workpiece is Al sheet.
 4. A method as claimed in claim 1, whereinthe electrolyte is at a temperature of 80-100° C. and a.c. anodising iscontinued for 0.1-10s at a current density of 0.1-10 kAm⁻².
 5. A methodas claimed in claim 1, wherein any oxide film on the cleaned surface ofthe workpiece is no more than 10 nm thick.
 6. A method of cleaning an Alor Al alloy workpiece which method comprises anodising the workpiece ata chosen a.c. voltage in an acidic electrolyte containing phosphoric orsulphuric acid capable of dissolving aluminium oxide and maintained at atemperature of at least 80° C. for high magnesium alloys and at least85° C. for all other alloys under conditions such that the surface ofthe workpiece is cleaned with any oxide film thereon being non-porousand having a thickness of not more than about 20 nm.
 7. A method asclaimed in claim 6, wherein the anodising is continued until anequilibrium is reached between oxide film formation and dissolution. 8.A method as claimed in claim 6, wherein the workpiece is Al sheet.
 9. Amethod as claimed in claim 6, wherein the electrolyte is at atemperature of 80-100° C. and a.c. anodising is continued for 0.1-10s ata current density of 0.1-10 kAm⁻².
 10. A method as claimed in claim 6,wherein any oxide film on the cleaned surface of the workpiece is nomore than 10 nm thick.